In the course of the 20th century the human race, without intending anything of the sort, undertook a giant, uncontrolled experiment on the earth. In time, according to John McNeill in his new book, the environmental dimension of 20th century history will overshadow the importance of its world wars, the rise and fall of communism, and the spread of mass literacy. Contrary to the wisdom of Ecclesiastes that "there is nothing new under the sun", McNeill sets out to show that the massive change we have wrought in our physical world has indeed created something new. To a degree unprecedented in human history, we have refashioned the earth's air, water and soil, and the biosphere of which we are a part. The author work is a compound of history and science. He infuses a substrate of ecology with a lively historical sensibility to the significance of politics, international relations, technological change and great events. He charts and explores the breathtaking ways in which we have changed the natural world with a keen eye for character and a refreshing respect for the unforeseen in history. He introduces us to little-known figures like Thomas Midgely, the chemical engineer who, McNeill claims, has had more impact on the atmosphere than any other organism in earth history. From Midgely's work with General Motors came the inventions of leaded gasoline and of Freon, the first of the chlorofluorocarbons that drift into the stratosphere and rupture ozone molecules. McNeill recounts episodes of environmental disaster - the mercury poisoning of Japan's Minamata Bay, the death of the Aral Sea in Soviet Central Asia - but shows too the successes of environmental policy in reversing pollution of the air and water. He fashions his story without pronouncements of doom or sermons on the ethical lapses of humankind. The author assesses the ecological course we have taken in the 20th century as an interesting evolutionary gamble. We have become exquisitely adapted to particular circumstances - a stable climate, cheap energy, rapid economic growth. But our fossil fuel-based civlization is on ecologically disruptive that it undermines the stability of these conditions. He does not speculate on the consequences, but his insights illuminate the new path we made in the global century.